|
From a discussion with milkie:
- This ticket originated with the mmap storage engine where fragmentation level had a large effect on performance; as long as there's disk space available, fragmentation generally won't affect WiredTiger performance.
- WiredTiger does have per-file fragmentation statistics.
- WiredTiger files require roughly the base size of the data that's not changing frequently, plus 2x the data changing between each checkpoint (there are normally two checkpoints in each file). If there is a lot of base data that's not changing rapidly, the file will tend to be closer to the table's data-set in size; if there's little base data and a lot of data is changing frequently, then the file will tend to be closer to 2x the data-set in size.
- If a user was seeing excessive file fragmentation, they might tune WiredTiger by increasing checkpoint frequency, or switching from the default best-fit allocation policy to a first-fit block allocation policy, or changing the file's block size, but MongoDB doesn't export those knobs, so currently there's nothing the user can do to adjust fragmentation.
Closing as won't-fix.
|